1. Field of the Invention
The present invention relates to heated windows, and more particularly, to heated windows for backlights in automobiles wherein the heating elements are dark colored electroconductive circuits secured to a surface of the windows.
2a. Technical Considerations
The deposit of moisture and ice on a automobile window has annoyed automobile owners considerably. Automobiles that are parked overnight are dangerous to drive until visibility is attained by removing the moisture deposit. In the past, this removal has been accomplished by applying a squeegee or scrapper to the surface of the window. Hot air blowers have also been employed to blow hot air across the surface of a fogged window. However, time is required to heat the air that is blown across the surface of the window to a temperature that is sufficient for the hot air to perform efficiently in the defogging or deicing function.
In addition, the inner surface of automobile windows tends to fog whenever the windows are closed and the moisture from the breath of occupants condenses on their inner surfaces. This source of fogging has also been difficult to remedy using the devices described above.
As an alternative to scraping, resistance wires have been attached to the surface of monolithic glass sheets or laminated glass plastic windows or embedded within a plastic interlayer of laminated windows to heat the window by passing electric current through the resistance wires connected between spaced bus bars. When a potential difference is applied between the bus bars, the heating wires convert the electric energy into sufficient heat energy to defog or deice the window as required.
It is common practice throughout the automotive industry to use silver ceramic paste which adheres to the glass surface to form the electroconductive heating circuits required to heat an automotive window. The pastes are generally silkscreened onto a glass sheet in a pattern that generally includes thin parallel horizontal lines connected at their ends near the edge of the window by wider bus bars. The silver ceramic materials provide sufficient electrical resistivity in the thicknesses and widths at which they are applied such that current to the electric circuit from an automobile 12 volt electrical energy system, will cause the circuit to heat and thus defog or deice the window within a reasonable amount of time.
The color of typical production silver ceramic pastes after being fired onto clear float glass, when viewed through the glass, ranges from a yellow to a medium brown color depending on whether the paste is applied to the air or tin side of a sheet of float glass. This color is slightly darker when viewed through tinted float glass. It is believed that this color results from the ionization of the silver of the paste during heating, and little or no reduction of these silver ions. It is the desire of automobile stylists, for cosmetic and aesthetic effects, to have a darker grid line color for the electroconductive heating elements.
2b. Patents of Interest
U.S. Pat. Nos. 3,467,818 to Ballentine and 3,659,079 to Whittemore, teach an electrically heated window panel wherein the glass surface silkscreen with thin lines of silver ceramic paste that are connected by bus bars. The typical ceramic conductive coating material includes a highly conductive metal powder, such as silver, in a vitrifying binder. The binder consists of lead borosilicate glass and a carrying medium.
U.S. Pat. No. 3,623,906 to Akeyoshi et al. teaches an electrically heated rear window for a motor vehicle wherein a plurality of parallel strips of an electroconductive frit are fired onto a glass window and electrically connected together by bus bars. The frit consists of finely divided electrically conductive metals such as silver, gold, copper or platinum, a low temperature melting powdered glass, such as lead silicate glass, and an organic binder.
U.S. Pat. No. 3,900,634 to Plumate et al. teaches an electrically heated panel with electrically conductive strips of paste having silver particles intermixed with a liquid vehicle and glass particles having a softening point lower than that of the glass substrate. The silver and glass particles are below five microns in size. The glass particles include two different glass compositions.